Fish Can Count, Study Finds

When it comes to counting, fish do swimmingly, according to a new study that describes the first published evidence of such behavior.

Captive mosquitofish—a North and Central American freshwater fish named for its taste for mosquito larvae—successfully counted geometric shapes in recent laboratory experiments.

Ten of the naturally social fish were first trained inside their tank to associate a door permitting them to move into different compartments and join their larger group with a certain number of shapes.

The same fish were then tested several times in an otherwise empty, unfamiliar tank to see whether they would choose to swim through the door marked with the right number of shapes.

The results showed that the fish chose the correct door more often than by chance alone, said study lead author Christian Agrillo, a psychologist at the University of Padova in Italy.

To make sure the fish weren't using non-numerical cues—for instance, estimating how much space objects take up—the Agrillo and colleagues placed sets of shapes that varied in size, brightness, and distance.

The sets were randomly selected so that only the number of shapes stayed the same.

"Only [these] kinds of studies may permit us to definitively understand whether an animal is counting, or, in contrast, using other quantitative mechanisms," Agrillo said in an email.

Although mosquitofish now join humans, monkeys, and other animals known to count, the ability in fish is probably a "last resort" strategy that has evolutionary underpinnings, Agrillo said.

That's because non-numerical cues probably come more easily to fish as they make rapid-fire decisions.

Being able to count may require more brainpower than simply judging numbers based on size. But counting might sometimes be necessary as the fish seek safety in numbers to shield themselves from predators, Agrillo said.